Currently, there are several industrial methods for covering an article by the melting of a polymer.
The first method is electrostatic powder coating, which consists in charging the powder with static electricity and bringing it into contact with the article to be covered, connected to zero potential. For example, the powder is injected in an electrostatic spray gun that charges the said powder by the corona effect, by triboelectrification or by a combination of both these. The powder thus charged is sprayed onto the article to be covered, which is connected to zero potential. According to another form of electrostatic powder coating, the article connected to zero potential is dipped in a fluidized bed of charged powder. Inside the bed is powder with which it is desired to cover the article. This powder is in the form of solid particles of small size, for example between 0.01 and 1 mm, of any shape, which are in a state of fluidization inside the bed owing to the presence of air or any other gas. The powder may be charged by electrodes, by the corona effect or by any device placed inside and/or outside the fluidized bed, in order to charge the powder by the triboelectric effect. The article covered with powder is then placed in an oven at a temperature high enough to provide a coating, by the powder melting and causing a film to form. For example, for a nylon-11 powder, it is sufficient to heat to 220° C.
The second method consists in preheating the article to be covered to a temperature above the melting point of the powder. Once hot, the article is immediately immersed in a fluidized bed of the powder, the powder melts on contact with the hot article and a film forms. A solid coating is thus provided. This method is also called “fluidized-bed dip coating”. As a variant of this last method, the powder can be sprayed onto the hot article.
The powder paint of the present invention is useful in both processes.
The invention relates to the field of high-performance coatings. The purpose of these is generally to provide the protected surface with many properties, whether they be for appearance or for providing a function (corrosion, abrasion, chemical, heat and UV resistance, low coefficient of friction, etc.).
Bacteria are single-cell organisms that reproduce by division. They are a few microns in size. There are thousands of species of bacteria, approximately 200 of which are pathogenic for humans. A distinction is made between Gram+ bacteria such as Staphylococcus aureus and Gram− bacteria such as Escherichia coli, Pseudomonas aeruginosa and Legionella sp.
Bacteria are transmitted through the air or by contact with the surface of articles. Transmission occurs in particular in public places, on means of public transport and in hospitals (nosocomial diseases). One way of combating these microorganisms is to protect surfaces from the appearance or growth of bacteria. This is generally done by using detergents. However, experience shows that repeated cleaning does not prevent many infections in hospitals. By obtaining antibacterial coatings it should be possible to keep surfaces clean between two cleaning operations and therefore to minimize the risks of contamination.
The incorporation of organic biocides into coatings makes it possible to obtain surfaces that are effective against bacteria of the Gram+ and Gram− type (Patent Application WO 97/46626). In most cases, the biocides used are of organic type. Their inferior thermal stability at 300° C. prevents them from withstanding the very severe conditions associated with the processing of polyamides. In addition, since their activity is obtained by migration of the biocide through the polymer, this rapidly decreases over time.
A new generation of inorganic biocides have been introduced. These compounds are primarily supports of the zeolite, glass or ceramic type or silver-exchange zirconium phosphate complexes. In the presence of moisture, ion exchange takes place, which releases silver ions that are powerful biocides. Their introduction into polymers so as to obtain powder coatings having antibacterial properties has been disclosed in the prior art (U.S. Pat. No. 6,432,416 and Patent Application JP 06025561 A). This type of biocide does not migrate, which guarantees that the surface has antibacterial properties throughout the life of the polymer. It is known that these products can be used in many applications, such as for example refrigerator shelves (Patent Application WO 02/40180) or grilles used in cooking (Patent Application WO 00/51429).
However, since silver ions are very reactive, this type of biocide may easily interact with the polymers to which they have been added and also with the additives and fillers possibly contained in these polymers, and cause discoloration problems. It is therefore very important for them to be perfectly stabilized. It is the nature of the inorganic support that fulfils this role, and which protects the silver species from the action of temperature or UV light. It also controls the rate of release of the silver ions. This rate must be high enough to ensure antibacterial activity that is effective but controlled, so as to avoid the release of silver into the polymer matrix and therefore the resulting interactions. In the case of polyamide-based powder paints, the processing temperatures, especially when they are applied by dipping, make the choice of biocide very difficult. Most inorganic compounds (silver-exchanged zeolites or silver-exchanged zirconium phosphates, for example) result in substantial and undesirable colour changes on the surface. Patent Application WO 01/90259 discloses an antibacterial powder paint composition based on PA11 and on a mixed compound based on silver and zinc that does not have discoloration problems, the zinc compound very probably acting as a sacrificial anode.
It is now been discovered that it is much simpler and more effective to add silver ions encapsulated in a soluble ceramic to the polyamide powder. These silver ions encapsulated in a soluble ceramic are in the form of powder and all that is required is to dry blend it with the polyamide powder before producing the coating. Such powders have been disclosed, for example, in Patent Application JP 2001247726A. These powders are commercially available, sold by Sanitized AG under the name SANITIZED® BC A 21-41.